1. Field of the Invention
The present invention relates to a super-junction Schottky PIN oxide diode.
2. Description of the Related Art
Increasingly efficient current converters are becoming necessary with regard to issues associated with CO2 emissions. Examples include inverters for photovoltaic or automotive applications. To this end, highly blocking, low-loss, quickly switching power semiconductors are necessary. In addition to active semiconductor switches such as IGBTs or CoolMOS transistors, freewheeling diodes are also required. For high-voltage applications, PIN diodes made of silicon are generally used. PIN diodes have small conducting-state voltages and low blocking currents, and therefore have low forward losses and blocking losses. However, high switching losses, which occur as switch-off losses during current commutation, are disadvantageous.
High-voltage PIN diodes are PN diodes in which an undoped (intrinsic), and in practice usually weakly doped, i layer is present between the p region and the n region. The blocking voltage is taken on primarily by the weakly doped i region. The space charge region extends mainly in the weakly doped region. The doping concentration and the thickness of this weakly doped region are determined by the predefined breakdown voltage. A high breakdown voltage means a low doping concentration and a large thickness of this weakly doped region. For a 600-V diode, the doping concentration of the i layer is approximately 3·1014 cm−3, and the layer thickness is approximately 50 microns.
High-level injection occurs in PIN diodes during flow operation with a high current density. In the process, electrons and holes are injected into the weakly doped region, and the concentration of the injected minority charge carriers exceeds the doping concentration of the weakly doped region. The conductivity of the weakly doped region is greatly increased as a result, and the voltage drop therefore remains low in the weakly doped middle region. For high currents, the forward voltage remains low. In contrast, no increase in the charge carrier density takes place for majority carrier components such as Schottky diodes. The weakly doped region represents a large ohmic resistor at which a correspondingly high voltage drops in the flow direction.
The charge carriers (electrons and holes), which are injected into the weakly doped region in the flow direction during the operation of PN or PIN diodes, must first be removed during switching off before the diode is able to take over blocking voltage. Therefore, during an abrupt current commutation the current initially continues to flow in the blocking direction until the stored charge carriers are removed or drained. This current is also referred to as drain current or reverse recovery current. The magnitude and duration of the drain current are determined primarily by the quantity of charge carriers stored in the weakly doped region. The more charge carriers that are present, the higher the drain current. A higher drain current means a higher switch-off power loss. Integration of the switch-off current over time results in reverse recovery charge Qrr, which is an important variable for describing the switch-off power loss, and which should be as small as possible. Switching times and switching losses are high for PIN diodes. Schottky diodes (metal-semiconductor contacts or silicide-semiconductor contacts) provide an improvement in the switching behavior. In Schottky diodes, no high-level injection takes place during flow operation, and therefore draining of the minority charge carriers is dispensed with. Schottky diodes switch rapidly and with practically no loss. However, for high blocking voltages, thick semiconductor layers with low-level doping are once again necessary, which for high currents results in unacceptable high forward voltages. For this reason, power Schottky diodes implemented in silicon technology, despite good switching behavior, are not suitable for blocking voltages above approximately 100 V.
Published German patent document DE 197 40 195 C2 describes a semiconductor element which is consistently referred to below as a Cool Schottky barrier diode (SBD). In this diode, as the result of introducing doped, alternatingly arranged p- and n-conductive pillars with a Schottky contact, it is possible to reduce the resistance to practically any desired level. When the pillar width is reduced, the pillar dopings may be increased. The doping of the p pillars and n pillars is selected in such a way that all dopant atoms are ionized when a blocking voltage is applied. This principle is also referred to as the superjunction (SJ) principle. Since a certain minority charge carrier injection takes place via the p-doped pillars, the ideal switching behavior of a pure Schottky diode is not achieved, but is greatly improved over that of a PIN diode. However, the low forward voltage of the PIN diode is not achieved for high currents. The superjunction principle is described, for example, in Japanese Journal of Applied Physics, Vol. 36, pages 6254-6262.